The present invention relates generally to high power systems utilizing a remote power source and, more particularly, to a method and apparatus of remotely controlling a power source designed to deliver power to a remote device. The invention further relates to a power source whose operation is governed by control signals received across a power cable connecting a remote device to the power source. The invention, while not so limited, is believed to be particularly applicable to welding, plasma cutting, and induction heating systems. One exemplary system to which the present invention is applicable is a Metal Inert Gas (MIG) welding system having a power source and a remote wire feeder.
MIG welding, formerly known as Gas Metal Arc Welding (GMAW), combines the techniques and advantages of Tungsten Inert Gas (TIG) welding's inert gas shielding with a continuous, consumable wire electrode. An electrical arc is created between the continuous, consumable wire electrode and a workpiece. As such, the consumable wire functions as the electrode in the weld circuit as well as the source of filler metal. MIG welding is a relatively simple process that allows an operator to concentrate on arc control. MIG welding may be used to weld most commercial metals and alloys including steel, aluminum, and stainless steel. Moreover, the travel speed and the deposition rates in MIG welding may be much higher than those typically associated with either TIG welding, formerly known as Gas Tungsten Arc Welding (GTAW), or Shielded Metal Arc Welding (SMAW) also known as stick welding, thereby making MIG welding a more efficient welding process. Additionally, by continuously feeding the consumable wire to the weld, electrode changing is minimized and as such, weld effects caused by interruptions in the welding process are reduced. The MIG welding process advantageously produces very little or no slag, the arc and weld pool are clearly visible during welding, and post-weld clean-up is typically minimal, relative to other welding techniques. Another advantage of MIG welding is that it can be done in most positions which can be an asset for manufacturing and repair work where vertical or overhead welding may be required.
A wire feeder is operationally connected to the power source and is designed to deliver consumable wire to a weld. To further enhance the operability of the wire feeder of a MIG welding system, known welding systems have connected the power source and the wire feeder to one another across a dedicated control cable that is in addition to a dedicated power or weld cable. Control signals defining the operational parameters of the power source are transmitted or fed back from the wire feeder to the power source. This is generally referred to as remote control to the power source.
One type of remote control device is used to regulate the operational welding parameters, and switch the welding power source output ON and OFF as well as change the power source state via a pendant that connects to the power source by a multi-conductor cable. This solution is schematically illustrated in FIG. 1A. A wire feeder 2A is connected to a remote power source 4A by a control cable 6A that includes a 14-pin connector (not shown). The cable 6A is used to transmit operational information to, and in some cases, from the power source, and may incorporate two to fourteen conductors depending on the number of functions that are to be controlled. Separately connected between the power source 4A and wire feeder 2A is a high-current/high-voltage weld cable 8A that delivers welding power to the wire feeder and creates a voltage potential between an electrode and a workpiece that is exploited to create and maintain an arc between the electrode and workpiece during welding.
A significant drawback to this cable-based control is that the control cable is typically fragile relative to the welding cables designed to carry high currents at high voltages. Welding machines are commonly used at construction sites or shipyards where it is not uncommon for the welding machines to be periodically relocated or surrounded by other mobile heavy equipment operating in the same area. As such, the remote control cable can become damaged by being crushed or snagged from contact with surrounding machines and/or traffic. This can cause damage to the wire feeder and/or the welding power source if internal power conductors become shorted to signal leads that are connected to sensitive signal level circuitry.
Another known system is a voltage following or voltage sensing wire feeder having an internal contactor. This solution is schematically shown in FIG. 1B. As shown, this system includes a wire feeder 2B that receives its electrical power from the voltage present in the welding circuit. The wire feeder is connected to a remote power source 4B via a weld cable 8B. One disadvantage of this system is that the operator has no convenient way to adjust the output of the welding power source to compensate for changes in workpiece thickness and/or fit up. The operator may call another person more conveniently located to the power source with a radio or some other means of communication to make the adjustment; however, if the operator is working alone, s/he must return to the power source to make the necessary adjustments. Another disadvantage of this system is that it requires the presence of a high current DC contactor to de-energize the welding circuit at the wire feeder. These contactors are large, heavy, costly, and require periodic maintenance to ensure proper and continual operation. The location of the secondary contactor in the remotely located wire feeder also requires that the welding circuit from the welding power source to the wire feeder remain energized even when not welding so that power is available to the wire feeder and welding arc when the gun trigger is activated. Accordingly, an open circuit voltage at or near a weld potential remains present across the weld cables. The weld cables, however, can become damaged at a worksite resulting in an unwanted arc being formed between an exposed portion of the cable and an unexpectant ground.
Another remote control solution is described in U.S. Ser. No. 10/604,482, which is assigned to the Assignee of the present application. Notwithstanding the numerous advancements achieved with the invention of the aforementioned pending application, such a system relies upon pulse width modulation to remotely transmit operational data from a wire feeder to a power source across a weld cable. By using pulse width modulated signals to remotely control operation of a power source, the amount of data as well as variability in the types of data that could be transmitted between the wire feeder and a power source is limited. Further, with the system described in the aforementioned pending application, the wire feeder requires an internal DC power supply to power the electronics of the wire feeder. That is, the invention of the above-referenced application teaches the avoidance of an open circuit voltage between the wire feeder and power source. As a result, absent a DC power supply, the wire feeder cannot be minimally powered so as to communicate with the power source to initiate the welding process.
It is therefore desirable to design a welding system incorporating a communications network or link for communication of control command signals from a wire feeder across a weld cable to control or otherwise regulate operation of a remote power source. It would also be desirable to design a remote controlled welding system without needing a dedicated DC power supply disposed in a wire feeder. It would be further desirable to design a wire feeder that receives a low voltage DC input to maintain powering of wire feeder electronics when the wire feeder is in a non-welding, standby state via only the weld cable connections.